The invention relates to a method for producing structural elements that comprise a hollow profile made of continuous fibre-reinforced plastics material comprising a thermosetting matrix, and at least one load-application element, and to a structural element of this kind.
In particular in the case of obvious load paths, as exist for example in the case of diagonal ties/struts, torsional shafts and pressure lines, fibre-composite profiles have a number of advantages compared with metal profiles, for example with respect to the rigidity properties thereof and the lower mass thereof. In this case, there are in particular two challenges with regard to using the fibre-composite structures, specifically the configuration of the application of the load to the profile and the integration of the profile into a module, and also efficient and cost-effective manufacture of the profiles.
There are essentially two types of method available for manufacturing fibre-composite profiles comprising a thermosetting matrix: the pultrusion method and piece production, e.g. in the RTM method.
In this case, the pultrusion method allows for continuous, cost-effective manufacture of fibre-composite profiles, but load-application elements that are required have to be introduced retrospectively, for example by means of adhesion technology or by machining the profile. Adhesive connections, i.e. integral connections, between a load-application element and the fibre-composite profile have little resistance to ageing; moreover, checking and quality assurance of the adhesive connection are extremely difficult. The elements disclosed in DE 10 2006 039565 A1, DE 10 2004 021144 A1 or DE 102 49591 A1, in which the load-application elements are adhesively bonded to fibre-composite rods, are suitable only up to a limited load level. Machining of the fibre-composite profile is disadvantageously associated with fibre damage and a deterioration in the mechanical properties.
In the case of piece production, as described in WO 2005/105417 A1 for example, the load-application element can be integrally moulded onto the profile during the manufacturing process, leading to advantageous mechanical properties. However, the method is complex and expensive.
Continuous manufacture of fibre-composite profiles that are intended to be connected to a load-application element, also referred to as functionalisation of the profile, is possible only when the load-application element is retrospectively integrated into the fibre-composite profile. This means that the profile must be formable in order for an integral connection to be established between the fibre-composite profile and the load-application element. This presupposes that it is possible to soften the matrix system during functionalisation.
This is possible in the case of thermoplastic matrix systems. A method for producing structural elements comprising a thermoplastic matrix is described in DE 10 2014 004158 A1. However, in addition to the typical disadvantages such as lower rigidity, higher temperature-sensitivity, poorer chemical resistance and greater water permeability, semifinished products comprising a thermoplastic matrix also often have quality defects compared with semifinished products comprising a thermosetting matrix. Compared with thermosetting matrix systems having a similar glass transition temperature in each case, said semifinished products comprising a thermoplastic matrix generally require higher consolidation pressures and temperatures, making tool systems complex and comparatively expensive.